Summary: Allows users to override the GK status of prefetching. It is implemented as a Tri-State. "Unset" means that the local Flipper config won't take precedent over the GK setting and will leave it unchanged. If a user interacts with the setting, it gets persisted into the config and will from then-on override the GK, meaning that the user has an opt-in mechanism and if we open the GK to more people, they effectively have an opt-out.
Reviewed By: jknoxville
Differential Revision: D18008259
fbshipit-source-id: bdfde9a8b9acf43aa60c84800a7979a29a4e9364
82a253cf28